i'm currently in 8th grade. i play the clarinet. this has also happened to me too. i kind of stuttered on my breath. i couldn't get the note out and i stuttered while i was breathing. something that helped was using a metronome. for me, when i used it and played solos etc, i wouldn't have that stutter. But when im either tapping my toe or there's no metronome, i have that stutter.